Tomatillos, often mistaken for small green tomatoes, are a distinct fruit belonging to the nightshade family, Physalis philadelphica. They are a staple in Mexican cuisine, providing the characteristic tart, slightly acidic tang that forms the base of traditional salsa verde. Unlike their tomato cousins, tomatillos are encased in a distinctive papery husk that dries and splits as the fruit matures. Successful cultivation of this warm-season annual relies entirely on precise timing, beginning with the correct indoor start date for your seeds.
Calculating the Optimal Indoor Start Date
The tomatillo requires a lengthy growing season and is sensitive to frost, making an indoor head start mandatory in most climates. To determine when to sow seeds, identify your area’s average last spring frost date. This date serves as the fixed point for your planting timeline.
Tomatillo seeds should be started indoors approximately six to eight weeks before that average last frost date. This timing allows seedlings to develop into robust, young plants, reaching four to six inches in height with several sets of true leaves—the ideal size for transplanting. Starting seeds too early results in oversized or root-bound plants that struggle outdoors. Starting too late may not allow enough time for a substantial harvest before the first fall frost.
For quick and uniform germination, the soil temperature must be consistently warm, ideally maintained between 70°F and 85°F. This is best achieved using a dedicated heat mat placed beneath the seed trays. Seeds typically germinate within seven to fourteen days. Once seedlings emerge, the air temperature can be slightly reduced to the 65°F to 70°F range to encourage stockier growth.
Indoor Seed Starting Requirements
Initial planting requires a sterile, fine-textured seed-starting mix rather than standard garden soil, which can harbor pathogens. Tomatillo seeds should be planted shallowly, about one-quarter inch deep, into small cell trays or individual pots. Planting two to three seeds per cell and thinning to the strongest single seedling after germination ensures one vigorous plant per pot.
Providing sufficient light is necessary after germination, as a sunny windowsill is rarely adequate. Seedlings need 14 to 16 hours of bright light daily, best delivered by dedicated grow lights suspended a few inches above the plants. This strong, direct light prevents stretching (etiolation), which results in weak, spindly stems.
Watering should keep the soil consistently moist but never waterlogged. Overwatering leads to fungal issues, while dry soil halts growth. Many growers prefer to water from the bottom, allowing the soil mix to wick up moisture from a tray. Once the seedlings have their first true leaves, they should be transferred to a slightly larger container (potted up) to prevent them from becoming root-bound.
Hardening Off and Transplanting
Hardening Off
Before moving young plants outdoors, they must undergo “hardening off” to acclimate them to harsher external conditions. This process toughens the tender indoor growth against direct sunlight, wind, and temperature fluctuations, which prevents severe shock. Hardening off involves moving the seedlings outside for short periods, starting with just one or two hours in a shaded, protected location.
Over a period of seven to ten days, the duration of outdoor exposure is slowly increased. Plants are progressively moved into brighter sun and more exposed spots. This gradual introduction prevents sun scald and strengthens the stem structure against wind damage. Reducing the frequency of watering slightly during this time helps slow growth and prepares the plant for the less consistent moisture conditions of the garden.
Transplanting
Transplanting should only occur after all danger of frost has passed, generally one to two weeks after the average last frost date. The soil temperature must be reliably above 60°F for the heat-loving tomatillos to thrive. Tomatillos have a sprawling growth habit and require ample space, so plants should be spaced at least 24 to 36 inches apart in a location that receives full sun. Providing a cage or stake at the time of planting is recommended, as their indeterminate growth benefits from support to keep heavy, fruit-laden branches off the ground.